Ballooning for Cosmic Rays

Ballooning for Cosmic Rays

Astronomers have long thought that supernovas are
the source of cosmic rays, but there's a troubling discrepancy
between theory and measurements. An ongoing balloon flight over
Antarctica could shed new light on the mystery.

January
12, 2001 -- Hold out your hand for 10 seconds. A dozen electrons
and muons just zipped unfelt through your palm. The ghostly particles
are what scientists call "secondary cosmic rays" --
subatomic debris from collisions between molecules high in Earth's
atmosphere and high-energy cosmic rays from outer space.

Cosmic rays are atomic nuclei and electrons that streak through
the Galaxy at nearly the speed of light. The Milky Way is permeated
with them. Fortunately, our planet's magnetosphere and atmosphere
protects us from most cosmic rays. Even so, the most powerful
ones, which can carry a billion times more energy than particles
created inside atomic accelerators on Earth, produce large showers
of secondary particles in the atmosphere that can reach our planet's
surface. [more]

Above: Supernova explosions, like the one that created
the expanding Crab Nebula (pictured), may be the
source of galactic cosmic rays.

Where do cosmic rays come from? Scientists have been trying
to answer that question since 1912, when Victor Hess discovered
the mysterious particles during a high altitude balloon flight
over Europe. Galactic cosmic rays shower our planet from all
directions. There's no definite source astronomers can pinpoint,
although there is a popular candidate.

"Most researchers are betting that cosmic rays come
from supernova explosions," says Jim Adams of the NASA/Marshall
Space Flight Center. When massive stars explode they blast their
own atmospheres into space. The expanding shock waves can break
apart interstellar atoms and accelerate the debris to cosmic
ray energies. Cosmic rays are subsequently scattered by interstellar
magnetic fields -- they wander through the Galaxy, losing their
sense of direction as they go.

"It takes an awful lot of power to maintain the galactic
population of cosmic rays," says Adams. "Cosmic rays
that lose their energy or leak out of the Galaxy have to be replenished.
Supernovae can do the job, but only if one goes off every 50
years or so." The actual supernova rate is unknown. Observers
estimate that one supernova explodes somewhere in the Galaxy
every 10 to 100 years -- just enough to satisfy the energy needs
of cosmic rays.

But there could be a problem with the supernova theory, says
Adams.

"A supernova blast blows a bubble in the interstellar
medium that grows until the shock wave runs out of energy,"
he explained. "They can accelerate particles up to some
point, about 1014electron
volts (eV) per nucleon, but not beyond that. Below an energy
of 1014 eV, all of the different cosmic ray species
-- protons, helium nuclei, etc. -- should have the same kind
of energy spectrum: a power law with index around -2.7."

Left:
This log-log plot shows the flux of cosmic rays bombarding Earth
as a function of their energy per particle. Researchers believe
cosmic rays with energies less than ~3x1015 eV come
from supernova explosions. The origin of cosmic rays much more
energetic than that (above the "knee" in the diagram)
remain a mystery.

A "power law" spectrum is one that looks like a
straight line on a piece of log-log graph paper. In the energy
range ~1010 eV to 1014 eV, the supernova
theory of cosmic ray acceleration predicts that the power law
spectrum of protons should have the same slope as the power law
spectra of heavier nuclei (about -2.7).

The problem is when scientists compare the energy spectra
of protons and helium nuclei, the two don't resemble one another
as much as they should. Both are power laws, as expected, but
"existing data indicate a possible spectral index difference
between protons and helium of about 0.1," says Eun-Suk Seo,
a cosmic ray researcher at the University of Maryland. "The
[slope of the] proton spectrum is close to -2.7, but the energy
spectra of helium and heavier nuclei seem to be flatter. The
difference is small and it might not be statistically significant."
If there is a genuine discrepancy, she added, it could signal
trouble for supernova models of cosmic ray acceleration.

To find out if the supernova theory is indeed in peril, a
team of scientists led by John Wefel ( Louisiana State University)
and Eun-Suk Seo, and aided by personnel from the National Science
Balloon Facility, launched a helium-filled
balloon from McMurdo, Antarctica on Dec. 28, 2000. The payload,
which is now 120,000 feet above Earth's surface, includes a NASA-funded
cosmic ray spectrometer known by its builders as the Advanced
Thin Ionization Calorimeter or "ATIC" for short.

"ATIC is sensitive to cosmic rays with energies between
~1010eV and 1014eV," says Wefel. By
covering such a wide range of energies with a single modern spectrometer,
the team hopes to measure the proton and helium cosmic ray spectra
with better precision than ever before.

Right:
The ATIC payload hangs from a launch vehicle while the helium
balloon is being filled in the background by personnel from the
National
Science Balloon Facility. The ATIC experiment lifted off
on its circumpolar trip to measure Galactic cosmic rays on Dec.
28, 2000.

"The higher energy cosmic rays are rare," he continued.
"For example, each day ATIC collects no more than ~10 cosmic
rays with energies exceeding 1013 eV. That's why we
have to fly the balloon for such a long time, to gather enough
particles for a statistically significant result." By the
time ATIC lands on January 12th or 13th, the spectrometer will
have been in the stratosphere counting cosmic rays for nearly
two full weeks.

The long flight time, more than any other reason, is why the
researchers chose to fly the balloon over Antarctica. "We
would be happy to fly this payload over North America,"
says Adams. "The problem is that we need the spectrometer
to be aloft for a long time. Antarctica has two advantages: It's
international territory, so we don't need to apply for lots of
overflight permissions, and the Antarctic Vortex (a circulating
weather system around the south pole) keeps the balloon confined
to airspace over the continent."

"If there is a difference between the proton and the
helium spectra -- and that's not certain, by the way -- it won't
necessarily kill the supernova model," continued Wefel.
"But a discrepancy would cause problems." Theorists
may have to consider the progress of supernova shock fronts in
greater detail. "Every supernova explosion is an individual
work of art," says Adams. "We use mathematical models
that assume the explosions are spherical, but they are not. Within
the blast wave itself you can see irregularities. There are bright
knots, for example, where shock waves run into interstellar clouds.
In crowded groups of massive stars ('OB associations') where
supernovae can occur in quick succession, blast waves collide
with other blast waves." It can get a little messy! Modeling
such details might affect any necessary reconciliation between
the theory and the data.

Above: The ATIC balloon payload. Click on the image
to find out how the Advanced Thin Ionization Calorimeter works.

And what if the supernova model can't be rescued? "There
are other possibilities," says Wefel, "but not a lot
of good ones. We'll really have to look hard to find something
other than supernovae that can meet the cosmic ray energy requirement."

The analysis team led by Eun-Suk Seo is eager to sift through
ATIC's data files after the balloon lands. The new particle counts,
which the experimenters hope will be the most accurate to date
in ATIC's energy range, could shed new light on the decades-old
mystery of cosmic rays.

Visit the ATIC home page for status
reports about the ongoing balloon flight. Participants
in the ATIC project include Louisiana State University, the University
of Maryland, NASA, the Naval Research Laboratory, Southern University
(Baton Rouge), the National Science Foundation, and collaborators
from Germany, Korea and Russia.

The Science and Technology Directorate at NASA's
Marshall Space Flight Center
sponsors the Science@NASA web sites. The mission of Science@NASA is to
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